1. Field of the Invention
This invention relates to an apparatus and method for cleaning semiconductor substrates or other such items requiring extremely high levels of cleanliness.
2. Description of the Related Art
Semiconductor substrates can be cleaned by propagating acoustic energy, such as megasonic energy, into a layer of cleaning solution on the surface of the substrate. Megasonic cleaning systems use this cleaning solution layer to propagate megasonic energy, i.e. acoustic energy at frequencies much greater than ultrasonic. This energy is directed toward the surface of the substrate and thereby removes, safely and effectively, particles from the substrate surface without the negative side effects associated with ultrasonic cleaning.
In the past, such cleaning systems have been designed to process substrates in batches, typically cleaning 25 substrate at once. The benefit of this batch cleaning became less important as substrate size increased because single substrate capacity increased. Also, substrate processors began working with more delicate devices, which required more careful handling than was possible in batch cleaning. The greater value per substrate and the more delicate nature of the devices produced on the substrates created a great need for single wafer processing equipment.
Single substrate megasonic cleaning equipment for processing the larger substrates carrying more delicate devices have been developed to meet this need. One such single substrate cleaning system incorporates a probe and a transducer and is described in U.S. Pat. No. 6,140,744 and commercially available from Verteq Inc. of Santa Ana, Calif. One cleaning apparatus described therein comprises an elongate probe configured to propagate megasonic energy to a surface of a substrate by way of a meniscus of liquid extending between the probe and the substrate. Because the energy is transmitted through a meniscus of liquid, the process is a “wet” process and it requires the probe to be positioned very close to the substrate surface.
After this “wet” cleaning process, the substrate must be dried prior to further processing. Various methods of drying the substrate have been tried and have generally involved spinning the substrate and thereby forcing the liquid off the substrate surface via centrifugal forces arising from the spinning. Unfortunately, this drying method has its drawbacks, such as the tendency of liquid on a surface to leave behind residue, e.g. water spots. In the past, such spots were not of great concern to the simpler devices being produced on the substrates. However, as already mentioned, the devices processed on substrates have become more delicate, and therefore more sensitive to contaminants of all kinds, including water spots. Moreover, substrate processors have become more aware of sources of process variation, which translate into variation in performance of the devices and yield variation. One such source of these variations is contaminants, including drying residue. Therefore, careful control of the drying conditions has been investigated by some.
European patent application publication EP0905747A1 to IMEC discloses a drying apparatus that exploits rotational and Marangoni effects to improve drying performance. As mentioned above, the rotation of the substrate subjects the liquid to centrifugal forces, which tend to force the liquid from the center of the substrate toward its edge, and ultimately off of the surface. Simultaneously, a surface tension reducing vapor creates the so called Marangoni effect that reduces the tendency of the liquid to adhere to the substrate surface, i.e. reduces the liquid surface tension. This reduces the tendency of the liquid to remain on the substrate surface long enough to evaporate from the surface and therefore helps to produce a residue free drying process. While the IMEC apparatus has achieved satisfactory substrate drying results in the laboratory, the concept has not been implemented into a commercial application.
Another issue presented by wet spin cleaning and drying of substrates is the containment and disposal of the process liquids involved, for example, various acids, bases, solvents, and de-ionized water. Some of these liquids may harm workers or damage other equipment in the vicinity of the cleaning apparatus if the workers or equipment come into contact with the process liquids. Thus, full containment and removal of the process liquids is necessary to maintain a safe working environment and protect valuable equipment.
However, a critical design consideration for any machine in substrate processing is process time, or through-put. This is in part because substrate processing must be done in very clean, and thus very expensive, fabrication facilities. As a result, substrate processors prefer to maximize the output of existing facilities rather than expanding those facilities or building new ones. Thus, fast through-put is preferred.
Therefore, a need exists for an improved cleaning method and apparatus that will improve the drying performance in a single wafer processing application and will improve throughput for performing substrate cleaning and drying operations.